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Characterization of Semiconductor Laser Gain Media by the Segmented Contact Method
Abstract
In this paper, we describe methods for analysis of edge-emitted amplified spontaneous emission spectra measured as a function of the pumped stripe length. We show that both the modal gain and the unamplified spontaneous emission spectra can be extracted from the data, and we describe a means of calibrating the spontaneous emission in real units, without requiring the carrier populations to be described by Fermi functions. The gain and emission spectra can be determined for transverse electric and transverse magnetic polarizations and by summing the recombination currents for each polarization the total radiative current can be measured. This enables the overall internal radiative quantum efficiency to be calculated. Once the calibration factor is known the internal stimulated recombination rate at the facet can also be estimated. The experiment can be configured to give a measurement of the passive modal absorption of the gain medium. The internal optical mode loss can be determined from the long-wavelength region of the gain spectrum or the modal absorption spectrum. In summary, we show that measurements of amplified spontaneous emission spectra provide a full characterization of the gain medium.
... The modal absorption is a map of available transitions between energy states (ground state and excited state) in the QD material when the device is un-pumped. The modal absorption of the samples was measured at room temperature using a segmented contact method [22]. In this method, the amplified spontaneous emissions (ASEs) of the multi-section non-laser devices are collected for the front section (I ASE (1) ) and back section (I ASE (2) ), separately under high-driven current. ...
... In this method, the amplified spontaneous emissions (ASEs) of the multi-section non-laser devices are collected for the front section (I ASE (1) ) and back section (I ASE (2) ), separately under high-driven current. The modal losses (A+ α i ) were calculated from Ref. [22]: ...
... Where ∮ A(E) d(E) is the area under the curve of the modal absorption, and N dots represents the number of dots density which was estimated from the TEM images of the samples as; 2.28 x 10 10 , 1.81 x 10 10 , and 1.33 x 10 10 cm − 2 for 8, 16, and 24 nm barrier width samples, respectively. w mod is the optical effective mode and it can be calculated from w mod = Lz Γ [22], where L z is the quantum well width (8 nm) and Γ is the confinement factor (0.02) for these samples. These results indicate that by adapting the barrier matrix width in InAsP-QD, it is possible to modify the inhomogeneous broadening and optical absorption cross-section. ...
... The samples were fabricated into a 50 μm wide, a multi-section structure 2 mm length device of a 300 μm long section as shown in Fig. 3. Using the SCM, amplified spontaneous emission (ASE), optical gain, spontaneous emission and population inversion factor will be investigated as described in [31]. Furthermore, 2 mm cavity length lasers with un-coated facets were prepared for each sample. ...
... The net modal gain (G − α i ) was calculated via Equation (1) [31]: ...
... where L is section length (300 μm), I meas (L 1+2 ) is ASE when both section 1 and section 2 are pumped, I meas (L 1 ) is the ASE of section 1 and α i ia the internal optical losses. The spontaneous emission spectrum (I meas Sp ) can be calculated as [31]: ...
We examine the probability of carriers in a 770 nm InAsP quantum dot (QD) laser by analyzing Fermi-Dirac distribution using the segmented contact method. This study compares the results of InAsP QD materials with the standard 720 nm InP QD laser that was developed by Cardiff University. Both samples were grown under the same conditions on a compatible wafer. InAsP QD showed non-uniformity in the dot size with a higher strain field, as evidenced by transmission electron microscopy images, as well as a deeper dot confinement by approximately 100 meV, as confirmed by Photovoltage Spectroscopy. This study revealed a breakdown of the Fermi-Dirac distribution at 150 K for InAsP QD materials. This could make InAsP a capable applicant for the ultrafast source generation.
... After the growth of InP buffer layer on GaAs substrate, the full laser structures were grown by using TMIn, TMAl, TMGa, AsH 3 , and PH 3 with Si 2 H 6 and diethyl-zinc (DEZn) as n-doping and p-doping precursors with the growth parameters summarized in Table 2. The structure contained a 3-period In 0.65 Ga 0. 35 As / In 0.5 Al 0.26 Ga 0.24 As (7/14 nm) active region, In 0.53 Al 0.23 Ga 0.24 As separate confinement heterostructure (SCH), InP cladding layers, and In 0.53 Ga 0.47 As contact layers. The same complete laser structure, shown in Fig. 2, was also grown on the InP buffer layer, which employs InAs QD DFs, on Si substrate [18,19]. ...
... Further investigation on the formation of this V-pit and extracting its volumetric density remains as a future work. It is noteworthy that the presence of V-pits can possibly lead to the optical scattering within the laser cavity, increasing the internal optical loss, which, in turn, can result in a lower slope efficiency Performing internal parameter extraction techniques such as cavity length analysis [34] or segmented contact method [35], combined with additional larger area TEM imaging, will be necessary to experimentally identify the impact of these V-pits on the device performance. ...
... While the observed T 0 value (51K) from the device grown on InP MBL on GaAs substrate was similar to the value (52 K) obtained from the device grown on InP substrate, the devices grown on Si substrate exhibited a significantly lower slope efficiency (∼26.5 mW/A) and characteristic temperatures, T 0 ∼41K and T 1 ∼53K, together with a blueshift in the lasing wavelength. The STEM investigation revealed compositionally graded and / or rougher interfaces of In 0.65 Ga 0. 35 As QW active region grown on Si substrate, which was correlated to the presence of severe active region carrier leakage evident from the observed device characteristics. This observation by STEM and device characterization both suggest that further reduction in TDD as well as improvement in the interfacial abruptness are necessary to further improve the performance of the QW-based lasers grown on mismatched substrate. ...
We report the characteristics of the strained In0.65Ga0.35As triple quantum well (QW) diode lasers grown by metalorganic vapor phase epitaxy (MOVPE) on lattice-mismatched substrates such as GaAs or Si, by utilizing InP metamorphic buffer layers (MBLs) in conjunction with InAs nanostructure-based dislocation filters. As the lattice-mismatch between the substrate and InP MBL increases, higher threshold current densities and lower slope efficiencies were observed, together with higher temperature sensitivities for the threshold current and slope efficiency. Structural analysis performed by both high-resolution X-ray diffraction (HR-XRD) and transmission electron microscopy indicates graded and/or rougher QW interfaces within the active region grown on the mismatched substrate, which accounts for the observed devices characteristics.
... A series of detailed analyses of the electronic properties of GaAsBi alloys have been undertaken [100,101] to develop a comprehensive theoretical model for dilute bismide quantum well lasers to elucidate the effects of Bi incorporation on the electronic and optical properties of ideal GaAs-based laser structures operating at wavelengths up to 1.55 μm [87]. This theoretical model was used in conjunction with experimental studies of the spontaneous emission and optical gain spectra of the GaAsBi SQW laser structures characterised as presented earlier in this chapter and additionally by applying the segmented electrical contact method [102] allowing direct measurements and analysis of the absorption, gain and spontaneous emission spectra in GaAsBi laser structures [96]. Fig. 9 shows optical microscope images of the fabricated devices and the schematic diagram of the segmented contact approach. ...
... It is determined as the product (G=Γg) of the local material gain, g, and the optical confinement factor, Γ, defining the fractional overlap of the optical field intensity with the active QW layers. In presence of optical gain, the total amplified spontaneous emission (ASE) of the same polarisation emerging from a length l per unit stripe width is determined from the following equation (1) where Ispon is the total spontaneous emission (SE) rate uniform along the contact stripe and defined per unit photon energy and per unit area in the plane of the layer, β is the fraction of the SE coupled into the waveguide and αi denotes the internal or cavity optical losses [102]. From Eq. (1), defining C as an extraction factor and taking into account reflection at the end facet with reflectivity R, the externally measured ASE spectrum can be written as ...
... ( The segmented contact method is based on an analytical solution of the Eq. (1) for two lengths l=L and l=2L, consisting of one and two equal length segments (L) which together with Eq. (2) gives the following expressions for the net modal gain (G-αi), and spontaneous emission rate, Ispon, [102] (4) Thereby, by measuring the ASE spectra from the device end under pumping of only the first section of length L, Imeas(L), (see Fig. 9 (c)) and from both pumped sections of equal length, Imeas(2L), and using Eqs. (3) and (4), we obtained and analysed the net modal gain and SE spectra, respectively. ...
In this chapter, we briefly review existing approaches for near- and mid-infrared lasers and show why III-V bismides are attractive as an alternative approach to conventional semiconductor material systems. We discuss a range of possibilities for practical applications for bismuth-containing semiconductor lasers that benefit from the additional flexible and effective control of energy bands and for the suppression of Auger recombination and inter-valence band absorption; the main processes limiting the performance of existing commercial lasers and responsible for significant energy usage. We discuss progress towards the final goal of temperature insensitive laser diodes and present a comprehensive set of data on the characterisation of GaAsBi lasers including optical gain and absorption characteristics and an assessment of the dominant carrier recombination processes in current state-of-the-art devices. We review the potential of GaAsBiN and InGaAsBi material systems for near- and mid-infrared photonic devices on GaAs and InP platforms, respectively, as well initial results on mid-infrared GaSbBi lasers grown on GaSb substrates.
... Optical gain measurement is a fundamental requirement for the characterization of lasers and amplifiers owing to This paper presents a comparative, empirical analysis of H&P [11], SC [12] and IA methods [13] for gain measurement. The analysis being presented here would mainly contribute towards the understanding of many body effects at high carrier/current densities and evolution of states at comparatively lower carrier/current densities. ...
... The schematic for pumping a multi-section device for SC gain measurement method [12] is shown in Fig. 4. In this case, the gain is evaluated via finding the ratio of the amplified spontaneous emissions obtained by pumping the sections of lengths L and 2L with the same current density. ...
... The SC gain measurement method is applicable to both multi-modal and single mode laser devices provided higher order modes are entirely eliminated by either spatial filtering [12] or an additional device length is left un-pumped [16] in the front of pumped sections. In our case the second option is used and an un-pumped section length of the order of 250μm is left at the front as an unguided spontaneous emission filter. ...
This paper presents a comparative analysis of three gain measurement methods which are H&P (Hakki & Paoli), SC (Segmented-Contact) and IA (Integrated-Amplifier) for the gain characterization of 1300nm (O-band) InAs/GaAs QD (Quantum Dot) laser devices. In this case, during continuous mode operation at a fixed heat-sink temperature of 17oC, the experimental conditions, measured spectral ranges and signal to noise ratio are compared and advantages are discussed. The devices used for the analysis are fabricated as multi-section, single mode structures. Before self-heating, each of the methods show identical results but SC proved to be better in terms of accuracy of internal loss measurement. The H&P method has been shown the only choice for high current density gain measurements at a fixed junction temperature under consideration. The method to remove self-heating effects via H&P is also discussed and via this method a high current density gain analysis upto 5.5kA/cm2(~ 8e-h pairs)is performed under 30 oC fixed junction temperature condition. In comparison to other methods, the IA method has shown to be advantageous in terms of low current density measurements exhibiting the capability of accessing wider spectral ranges and performing the gain characterization where laser material operates in loss.
... Several methods employing an integrated source were suggested to overcome this limitation. They are based on analysis of the amplified spontaneous emission (ASE) spectra from segmented SOAs, where the external source is replaced by one of the sections of the device [5], [6]. The The waveguide on the other side of the device should be designed in a way to ensure no returning light back into the device (e.g., terminated with an absorber). ...
... The The waveguide on the other side of the device should be designed in a way to ensure no returning light back into the device (e.g., terminated with an absorber). absorption can be calculated from the incident and transmitted light intensities for one absorber length [6], or from the fit of the transmitted light intensity for multiple absorber lengths [5]. A modification of the method was suggested by Xin et al. [7] in order to exclude the contribution of unguided spontaneous emission from the structure. ...
... In Section 3, we describe the proposed method for the absorption calculation. Besides this, we briefly describe the reference method from [6] which we use to verify the results. In Section 4 we describe the structure design and fabrication followed by the description of the measurement setup. ...
Photonic integrated circuits often use semiconductor components such as amplifiers, detectors, and electroabsorption modulators. For a proper circuit design, it is important to know the absorption spectrum of these semiconductor optical components and how it depends on an applied electric field. We propose a fast and accurate method that uses a compact segmented contact structure to measure the absorption characteristics. The method is based on measuring the transmission of amplified spontaneous emission (ASE) from a single forward-biased section through a varying number of reversely-biased absorbing sections. Provided the ASE source emits light in both polarizations, the method measures the absorption spectra for both polarization modes simultaneously, without the need for a polarization filter in the measurement setup.
... The absorption modal of the samples was measured using a multi-section device technique introduced in [22]. This technique involves measuring the amplified spontaneous emission (ASE) of the section 1 of the device and the ASE of the section 2 separately under specific pumped current. ...
... This technique involves measuring the amplified spontaneous emission (ASE) of the section 1 of the device and the ASE of the section 2 separately under specific pumped current. The modal absorption ( ) can be calculated as [22]: ...
In this report, we measured experimentally the modal absorption spectra of the InP and InAsP quantum dot (QD) lasers using multi-section device technique. The optical absorption cross section (σ0) and inhomogeneous broadening for the ground state (GS) and excited state (ES) were analyzed and calculated theoretically from the absorption spectra. The results showed that the InP QD laser exhibited σ0 to be 1.347×10−14cm2.eV and 3.016×10−14cm2eV for GS and ES respectively, whereas for the InAsP QD material it was found as 0.511×10−14cm2eV and 3.099×10−14cm2.eV for GS and ES respectively. Moreover, the inhomogeneous broadening in the GS increases from 35.6 eV to 63.6 eV when As was added to InP QD, similarly, the inhomogeneous broadening of ES increases from 46.9 eV to 103.8 eV. The alloying InP QDs with arsenic decreases the σ0 of the ground state (lasing state) and increases both inhomogeneous and linewidth broadenings. This finding may help the grower to control the growth conditions and the molecule fractions of the crystal to improve the spectral properties of the optoelectronics devices.
... n th is determined using the experimentally determined losses, where available. We note that the losses are most reliably determined from Hakki-Paoli [35] or segmented contact methods [47], as we use here. Inverse external differential efficiency measurements versus cavity length measurements while often reported are prone to error owing to carrier density non-pinning effects [18]. ...
... Nonpinning of the spontaneous emission intensity above threshold has often been observed in both type-I [18,51] and type-II [59, 60] mid-IR lasers, as well as in structures affected by inhomogeneity of the size or composition of the active region [51,61]. Therefore, as an alternative we have measured losses by the segmented contact method [47]. Using this approach, we measured internal losses of approximately 10 cm −1 at wavelengths close to 2.5 µm, as shown in figure 4, which presents the net optical absorption (A + α i ) spectrum at room temperature. ...
From a systematic study of the threshold current density as a function of temperature and hydrostatic pressure, in conjunction with theoretical analysis of the gain and threshold carrier density, we have determined the wavelength dependence of the Auger recombination coefficients in InGaAsSb/GaSb quantum well lasers emitting in the 1.7–3.2 µ m wavelength range. From hydrostatic pressure measurements, the non-radiative component of threshold currents for individual lasers was determined continuously as a function of wavelength. The results are analysed to determine the Auger coefficients quantitatively. This procedure involves calculating the threshold carrier density based on device properties, optical losses, and estimated Auger contribution to the total threshold current density. We observe a minimum in the Auger rate around 2.1 µ m. A strong increase with decreasing mid-infrared wavelength (<2 µ m) indicates the prominent role of intervalence Auger transitions to the split-off hole band (CHSH process). Above 2 µ m, the increase with wavelength is approximately exponential due to CHCC or CHLH Auger recombination, limiting long wavelength operation. The observed dependence is consistent with that derived by analysing literature values of lasing thresholds for type-I InGaAsSb quantum well diodes. Over the wavelength range considered, the Auger coefficient varies from a minimum of ≲ 1 × 10 ⁻¹⁶ cm ⁴ s ⁻¹ at 2.1 µ m to ∼8 × 10 ⁻¹⁶ cm ⁴ s ⁻¹ at 3.2 µ m.
... To further investigate the internal parameters in the semipolar LDs on sapphire, segmented contact characterization was carried out by measuring the net modal gain (Γg − ⟨α i ⟩) spectra through injecting currents onto separate segments of the p-contacts. 31,32 Such structure is comprised by two 300 μm contacts. The spectra of amplified spontaneous emission under different applied currents are shown in Figure 6. ...
... Details of the setup and measurement for the loss can be found in the literature. 32 An internal modal loss (⟨α i ⟩) 32 cm −1 was obtained, which is higher than the blue LDs on semipolar bulk GaN substrate with a value around 10 to 20 cm −1 . 33,34 ...
Growth of semipolar GaN laser diodes (LDs) on low-cost and large-size foreign substrates is crucial yet remains very challenging. In this study, we report the world’s first continuous-wave (CW) electrically driven semipolar blue LDs at room temperature heteroepitaxially grown on a 4-inch sapphire substrate. The semipolar (202 ̅1) GaN material grown on sapphire substrate exhibits high crystal quality from the x-ray diffraction and transmission electron microscopy measurements. The AlGaN-cladding free semipolar blue LDs utilizing thin p-GaN/indium tin oxide (ITO) cladding structure show a lasing peak at 456 nm, a threshold current density of 6.1 kA/cm2, and a high output power (Po) of 1.03 W at 2.8 A under pulsed operation condition. Moreover, the semipolar blue LDs exhibit a CW Po of 243 mW at room temperature. The emission light from the semipolar blue LDs is 100% polarized. These results mark a significant breakthrough in substantially reducing the cost of semipolar LDs and expediting the development of future semipolar GaN LDs and their applications.
... Several methods may be adopted for measuring the gain such as the Hakki-Paoli method [9], its improved version called the Fourier Transform method [10] and the segmented contact method [11]. Whereas the first two techniques limit the gain measurement up to the laser threshold, the last one is preferred because it provides gain over a larger current range; in addition, the internal modal loss, the absorption and the spontaneous emission spectrum can also be measured [12] . The fitting of the measured absorption spectrum is very useful to extract the dipole moment [13] that enters in the modeling expression (1.1). ...
... 12: ASE out of a QD-SOA studied with a TDTW model: (a) calculated (squares) and measured (solid line) L-I curve and (b) calculated (solid line) and measured (dashed line) ASE spectra[87]. ...
... A dot areal density of 5 × 10 10 cm −2 and mean height of 5 nm was determined by atomic force microscopy (AFM) of an uncapped sample, with nominal modulation doping corresponding to 10 holes per dot (hpd). Samples were fabricated into segmented contact devices with modal absorption measured using the segmented contact method (Blood et al. 2003). ...
A modelling routine has been developed to quantify effects present in p-modulation doped 1.3 μm InAs/InGaAs quantum dot laser and modulator devices. Utilising experimentally verified parameters, calculated modal absorption is compared to measurements, prior to simulation of structures under reverse and forward bias. Observed broadening and a reduction of absorption in p-doped structures is attributed primarily to increased carrier scattering rates and can bring benefit when structures are configured as optical modulators with enhancements in the figure of merit. However, increased carrier scattering limits the maximum modal gain that can be achieved for lasers. The state filling caused by p-doping only marginally reduces absorption but assists laser operation with increased differential gain and gain magnitude at lower current densities.
... In most current work discussing gain measurements of laser didoes, threshold gain, mirror loss, and internal loss are usually discussed in terms of whole reciprocal centimeters (cm À1 ). [14][15][16][17][18][19][20][21][22] We, therefore, set our convergence limits to be to within 60.1 cm À1 . However, this can be changed according to requirements. ...
Probabilistic Markov chain modeling of photonic crystal surface emitting lasers (PCSELs) is reported. This simulation links the scattering parameters of the photonic crystal (PC) and device level losses of the PCSEL. The criteria for the conversion of the numerical model and agreement with data from the literature are discussed. We then explore the effect of increasing PC coupling coefficients and boundary mirror reflectivity on the in-plane power loss from the PCSEL. The effect of spatially varying the boundary reflectivity on the near-field is also explored.
... 15,16 To study the performance of thin film materials as active media, several optical properties need to be quantified, such as their optical gain, amplified spontaneous emission (ASE) threshold, radiative recombination lifetime, etc. Characterization of optical gain is essential to benchmark and compare different materials as gain media, and to model the behavior of lasing structures using rate equations. Typically, optical gain is characterized using either transient absorption spectroscopy (TAS), 17 which provides information about the maximum intrinsic gain of the material, or the variable stripe length (VSL) 18−21 method, which measures the net modal gain of a material under optical pumping (or electrical pumping in the segmented-contact variant 22 ). Both techniques have been studied for decades 23,24 and have been extensively used to measure the gain for several material systems. ...
Accurate measurement of optical gain is essential to screen materials as viable active media for thin-film laser applications. The net modal gain is typically measured using the variable stripe length (VSL) method, which has been extensively studied for the last few decades. In this work, we propose an alternative method, which we name scattered emission profile (SEP) method, to measure the net modal gain. It relies on the collection of amplified spontaneous emission (ASE) scattered from the surface of the film illuminated by a pump stripe. By using an appropriate setup, the new method results in a significantly faster measurement of net modal gain, while simultaneously providing a more accurate gain value. The setup and algorithm to extract the net modal gain are detailed in this paper and are demonstrated on Lead Halide Perovskite films. The influence of the stripe length on the measured gain value is shown. Gain measurements performed over two different perovskite films, fabricated either via spin-coating or thermal evaporation, confirm the broad applicability of the SEP method. Finally, we show a quantitative comparison of the SEP method with VSL measurements, and highlight the advantages and shortcomings of each method.
... Here, we investigate the optical gain characteristics of VCSEL material using a stripe-length method [20] to measure the in-plane modal-gain as a function of carrier injection for a range of operating temperatures; an approach which, to our knowledge, has not previously been used on VCSEL material. Table 1 compares this work with previous work on the characterisation of active layers in VCSEL material. ...
We report direct measurements of the optical gain on vertical-cavity surface-emitting laser (VCSEL) material using a stripe-length method featuring segmented contacts. We utilise the similarity of the in-plane transverse electric (TE) polarised matrix element and that of the VCSEL lasing mode and a simple method to reduce round trip effects. The confinement factor is determined from cold-cavity simulations of the in-plane TE polarised slab waveguide mode and used to convert the measured in-plane modal gain into the vertical-cavity modal gain, as required for the VCSEL structure. This gives a threshold material gain of 1440 ± 140 cm ⁻¹ at 30 °C for this structure. A comparison with the threshold material gain values determined from the lasing condition, where internal optical losses due to doping induced absorption is included using parameters taken from the literature, indicates the presence of an additional source of optical loss in the experiment which increases the threshold material gain by ∼450 cm ⁻¹ . A best fit is obtained by increasing the optical loss in the n-DBR (distributed Bragg reflectors) layers to 40 cm ⁻¹ , which is consistent with previous work on additional scattering losses due to interface roughening in the n-DBR layers. To further demonstrate the utility of this method for rapid optimisation, the gain-peak wavelength is measured directly, and its temperature dependence is compared to the lasing wavelength.
... Electroluminescence (EL) spectrum was collected by an optical fiber connected to Ocean-Optics spectrometer, and the light output power was measured by integrating sphere. Segmented contact method was carried out to measure the optical loss utilizing the amplified spontaneous emission (ASE) spectra, where the method and principles can be found in [27,28]. ...
InGaN based c-plane blue LDs on strain relaxed template (SRT) with a reduced absorption loss was demonstrated. The loss is reduced from 27 cm−1 to 20 cm−1. Due to the lower loss, threshold current density is improved from 51.1 kA/cm2 to 43.7 kA/cm2, and slope efficiency is also increased by a factor of 1.22. The absorption loss from decomposition layer (DL) in SRT is confirmed to be a major extra loss source by both experimental and simulation results. With a higher indium content in buffer and waveguide layers, optical leakage into DL can be suppressed.
... The applied voltage and current were measured by the oscilloscope, and the light output power of the LDs was measured by an integrating sphere. The segmented contact method performed to measure the optical loss utilized the amplified spontaneous emission (ASE) spectra; the method and principles can be found in [39,40]. ...
Electrically driven c-plane InGaN-based blue edge emitting laser diodes on a strain-relaxed template (SRT) are successfully demonstrated. The relaxation degree of the InGaN buffer was 26.6%, and the root mean square (RMS) roughness of the surface morphology was 0.65 nm. The laser diodes (LDs) on the SRT laser at 459 nm had a threshold current density of 52 kA/cm2 under the room temperature pulsed operation. The internal loss of the LDs on the SRT was 30–35 cm−1. Regardless of the high threshold current density, this is the first demonstrated laser diode using the strain-relaxed method on c-plane GaN.
... Similarly, the gain and loss were found using the segmented contacts method. Described in detail by Blood et al. [27], two contacts in series on a ridge were alternately pumped measuring intensity and related to create gain and absorption curves, shown in Fig. 4(c) and Fig. 4(d), respectively. The absorption curve is asymptotic to the internal loss at long wavelengths and comes out to <α i > seg−ct = 45 /cm, reasonably close to previous calculations using the variable stripe. ...
GaN lasers with green emission wavelength at λ = 510 nm have been fabricated using novel nano-porous GaN cladding under pulsed electrical injection. The low slope efficiency of 0.13 W/A and high threshold current density of 14 kA/cm ² are related to a combination of poor injection efficiency and high loss, analyzed by the independent characterization methods of variable stripe length and segmented contacts. Continuous wave operation showed narrowed spectra and augmented spontaneous emission.
... The spectral dependence of the modal gain is obtained from ASE spectra (see Fig. 2.2 a)) using the stripe length method [94]. The measured ASE intensity I ASE can be calculated from the intensity of the spontaneous emission I SE via ...
Study of carrier dynamics in quantum dot amplifiers, e.g. small-signal gain, linewidth enhancement factor, dephasing times or self-induced transparency. Used methods are pump probe measurements, four wave mixing, heterodyne technique or frequency resolved optical gating.
... To further understand the polariton laser dynamics, and compare with conventional edge-emitting semiconductor lasers, we study the effect of the ratio of the pump length to the cavity length, in analogy with the segmented contact method for electrically injected ridge lasers [36], or the variable stripe-length method for gain measurement [37]. The gain is fed by the exciton reservoir in a polariton laser, so that the gain length can be estimated from the spatial profile of the exciton reservoir. ...
We experimentally demonstrate the difference between a ridge polariton laser, and a conventional edge-emitting ridge laser operating under electron-hole population inversion. The horizontal laser cavities are 20 -- 60 $\mu$m long GaN etched ridge structures with vertical Bragg reflectors. We investigate the laser threshold under optical pumping and assess quantitatively the effect of a varying optically-pumped length. The laser effect is achieved for an exciton reservoir length of just 15% of the cavity length, which would not be possible in a conventional ridge laser, with an inversion-less polaritonic gain about 10 times larger than in equivalent GaN lasers. The modelling of the cavity free spectral range demonstrates the polaritonic nature of the modes.
... Samples were fabricated into multi-section devices with absorption, modal gain and loss measured using the segmented contact method. Full details of the approach can be found in [10]. In the figures below we consider data from an undoped, sample 1, and p-doped, sample 2 (10nm thick doping layer 15nm above nearest dot layer), and sample 3 (10nm thick, 8nm above nearest dot layer), showing typical absorption data and calculated gain. ...
A semi-empirical model is compared with measurements to establish limiting factors in the performance of p-modulation doped InAs quantum dot (QD) lasers. Fitted absorption spectra allow identification of supposed factors and comparison of multiple samples isolates their origin, providing insights for future laser design.
... The segmented contact method was utilized to measure the QCSE in the QD active regions [6]. The active regions used are similar to those we have already measured in [5]. ...
The quantum confined Stark effect in InAs/InGaAs QDs using an undoped and p-modulation doped active region was investigated. Doping potentially offers more than a 3x increase in figure of merit modulator performance up to 100°C.
... Experimental measurements of optical gain and absorption in these devices are performed using the segmented contact technique developed by Blood et al [26]. Top stripe contacts are fabricated using a mask to produce segments 100 µm wide and 250 µm in length separated by 3 µm spacers [27]. ...
Type-II ‘W’-lasers have made an important contribution to the development of mid-infrared laser diodes. In this paper, we show that a similar approach can yield high performance lasers in the optical communications wavelength range. (GaIn)As/Ga(AsSb) type-II ‘W’ structures emitting at 1255 nm have been realised on a GaAs substrate and exhibit low room temperature threshold current densities of 200–300 A cm ⁻² , pulsed output powers exceeding 1 W for 100 µ m wide stripes, and a characteristic temperature T 0 ≈ 90 K around room temperature. Optical gain studies indicate a high modal gain around 15–23 cm ⁻¹ at 200–300 A cm ⁻² and low optical losses of 8 ± 3 cm ⁻¹ . Analysis of the spontaneous emission indicates that at room temperature, up to 24% of the threshold current is due to radiative recombination, with the remaining current due to other thermally activated non-radiative processes. The observed decrease in differential quantum efficiency with increasing temperature suggests that this is primarily due to a carrier leakage process. The impact of these processes is discussed in terms of the potential for further device optimisation. Our results present strong figures of merit for near-infrared type-II laser diodes and indicate significant potential for their applications in optical communications.
... A multi-section sample was used to measure the radiative recombination time in the InAsP QD laser. We used the segmented contact method discribed in [16]. The spontaneous emission spectrum can be calculated by means of: ...
In this paper, we applied the ABC model in quantum dot (QD) semiconductor laser for the first time. We used a 1000μm cavity length InAsP/GaAs quantum dot laser emitting at 761nm, which was improved at Cardiff University. The ABC model is used to estimate the carrier losses that are caused by spontaneous emission and Auger recombination in semiconductor materials. It is shown that the ABC model is applicable in such lasers. The results show that the Shockley-Read-Hall (A) is 2.03 ×l0 ⁹ sec ⁻¹ . The radiative coefficient (B) is 2.28 ×l0 ⁻¹⁴ cm ⁻³ .sec ⁻¹ and the Auger recombination (C) is around 8 ×l0 ⁻³⁷ cm ⁻⁶ .sec ⁻¹ . The results are very close to the actual findings as measured by several different methods. Moreover, the measurement method is feasible, which can pave the path for the use of this procedure to determine the losses mechanism in semiconductor lasers.
... Figure 17(a) shows a schematic top view of the segmented contacts structure and the tested device on the same sample, where LDs were fabricated. 98,99 The amplified spontaneous emission (ASE) spectra of Ca-ha i i and Cg-ha i i at various J are presented in Figs. 17(b) and 17(c), respectively. ...
III-nitrides based light-emitting diodes and laser diodes (LDs) have shown great success as solid-state lighting sources, but the development of common c-plane (0001) polar GaN emitters is facing limitations due to quantum-confinement Stark effect, efficiency drop, low efficiency at green range, and peak wavelength blue-shift. Efficient semipolar or nonpolar GaN light emitting diodes and LDs have been successfully demonstrated by growing on semipolar or nonpolar free-standing GaN substrates. The small size and high cost of high crystal quality semipolar or nonpolar free-standing GaN substrates, which are sliced from hydride vapor phase epitaxy grown c-plane bulk GaN substrate, have severely limited their commercial development and application. Achieving scalable heteroepitaxial semipolar GaN materials with a very low density of basal-stacking faults (BSFs) on a foreign substrate remains very challenging. The recent breakthrough in the demonstration of continuous-wave (CW) semipolar (20 2 ¯1) LDs at room-temperature on semipolar GaN/sapphire template marks a milestone in exploring high crystal quality heteroepitaxial semipolar GaN materials and low-cost semipolar emitters. Here, we review the key progress through the past years about the development of heteroepitaxial semipolar GaN materials including epitaxial lateral overgrowth, orientation controlling epitaxy, BSFs burying by neighboring Ga-polar (0001) GaN with air voids, facet-engineering orientation control epitaxy, resulting in a low density or free of basal stacking faults. Furthermore, we discuss the heteroepitaxially grown pulsed semipolar (11 2 ¯2) blue LDs and CW semipolar (20 2 ¯1) LDs.
... Experimental measurements of optical gain and absorption in these devices are performed using the segmented contact technique developed by Blood et al. [26]. Top stripe contacts are fabricated using a mask to produce segments 100 μm wide and 250 μm in length separated by 3 μm spacers [27]. ...
Type-II W-lasers have made an important contribution to the development of mid-infrared laser diodes. In this paper, we show that a similar approach can yield high performance lasers in the optical communications wavelength range. (GaIn)As/Ga(AsSb) type-II W structures emitting at 1255 nm have been realised on a GaAs substrate and exhibit low room temperature threshold current densities of 200-300 A cm^2, pulsed output powers exceeding 1 W for 100 μm wide stripes, and a characteristic temperature T0 ~ 90 K around room temperature. Optical gain studies indicate a high modal gain around 15-23 cm^-1 at 200-300 A cm^-2 and low optical losses of 8 +- 3 cm^-1. Analysis of the spontaneous emission indicates that at room temperature, up to 24% of the threshold current is due to radiative recombination, with the remaining current due to other thermally activated non-radiative processes. The observed decrease in differential quantum efficiency with increasing temperature suggests that this is primarily due to a carrier leakage process. The impact of these processes is discussed in terms of the potential for further device optimisation. Our results present strong figures of merit for near-infrared type-II laser diodes and indicate significant potential for their applications in optical communications.
... To perform the measurement of the net modal gain and absorption, the segmented contact method [12] was employed. Gold multi-section contacts were fabricated with a width of 100 m and a length of 300 m. ...
In this work, the feasibility of a monolithically integrated laser and electroabsorption modulator based on the same active quantum dot epistructure is studied. The net modal gain and the absorption in the modulator were measured using the segmented contact method from 25 °C to 125 °C. The maximum of the net modal gain active region of the laser decreases from 10 cm-1 at 25 °C to 3.9 cm-1 at 125 °C. The non-optimized maximum extinction ratio of the modulator, 4.1 dB·mm-1, is almost constant until 25 °C. The wavelengths at which the net modal gain and the change in absorption are maximum shifts with temperature by 0.04 eV.
... Cr/Au was deposited for the p-contact to a thickness of 10 and 400 nm respectively. Test structures to evaluate the optical loss, using the segmented contact method [20], are fabricated in a similar manner except that the top p-contact and underlying GaAs contact layer are etched to leave 4 μm wide gaps between contacts formed on a 300 μm pitch. These test structures can also be operated as a laser by driving all the sections in forward bias. ...
Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3-μm are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement, or cavity length. While carrier localisation in quantum dots minimises degradation, an increase in the number of defects in the early stages of ageing can increase the internal optical-loss which, can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the 2-D states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical-loss whereas the same structure grown on GaAs show no signs of increase in internal optical-loss when operated under the same conditions.
... Low internal differential quantum efficiency values obtained throughout this paper strongly suggest the growth optimisation for achieving a uniform Bi distribution in GaAs 1−x Bi x QWs, together with an optimised heterostructure employing high band-gap materials, is imperative to realise highperformance GaAs 1−x Bi x QW-based lasers. In addition, investigation of these internal device parameters by the other extraction techniques such as Hakki-Paoli method [37] or segmented contact method [38] will be necessary to quantify any errors originating from CLA measurements due to the possibility of an unclamped carrier concentration above laser threshold. ...
The impact of post‐growth thermal annealing on the internal device parameters such as internal loss (αi), internal differential quantum efficiency (η0d) and modal material gain (Γg0J) of a single‐quantum well (QW) laser diodes employing GaAs0.965Bi0.035/GaAs0.75P0.25 active regions and emitting near λ∼980 nm was investigated. Parameter extraction from a conventional cavity length analysis indicates that the internal loss remains unchanged while internal differential quantum efficiencies degrade as the annealing time increases. Also, the variation of the modal material gain with annealing correlates to the corresponding change in the photoluminescence intensity. Comparisons between single‐‐ and double‐QW devices indicate that the relatively high internal loss originates from the QW active region.
... While the external device parameters such as the change in the threshold current densities, slope efficiencies, and temperature sensitivities were investigated, additional studies are needed to extract internal device parameters including the material gain parameter and internal loss, by employing a multi-segment contact method. 42 H. Kim, Y. Guan, K. Forghani, T. F. Kuech, and L. J. Mawst, "Laser diodes employing GaAs 1Àx Bi x /GaAs 1Ày P y quantum well active regions," Semicond. Sci. ...
Laser diodes employing a strain-compensated GaAs1−xBix/GaAs1−yPy single quantum well (SQW) active region were grown by organometallic vapor phase epitaxy (OMVPE). High resolution x-ray diffraction, room temperature photoluminescence, and real-time optical reflectance measurements during the OMVPE growth were used to find the optimum process window for the growth of the active region material. Systematic post-growth in situ thermal anneals of various lengths were carried out in order to investigate the impacts of thermal annealing on the laser device performance characteristics. While the lowest threshold current density was achieved after the thermal annealing for 30 min at 630 °C, a gradual decrease in the external differential quantum efficiency was observed as the annealing time increases. It was observed that the temperature sensitivities of the threshold current density increase while those of lasing wavelength and slope efficiency remain nearly constant with increasing annealing time. Z-contrast scanning transmission electron microscopic) analysis revealed inhomogeneous Bi distribution within the QW active region.
We demonstrate the effect of arsenic composition percentage on inhomogeneous broadening, lasing wavelength and room temperature threshold current density. Lasing wavelength peak shows a red shift whereas inhomogeneous broadening became wider as As composition increased.
We experimentally demonstrate the difference between a ridge polariton laser and a conventional edge-emitting ridge laser operating under electron-hole population inversion. The horizontal laser cavities are 20–60-μm-long GaN etched-ridge structures with vertical Bragg reflectors. We investigate the laser threshold under optical pumping and assess quantitatively the effect of a varying optically pumped length. The laser effect is achieved for an exciton reservoir length of just 15% of the cavity length, which would not be possible in a conventional ridge laser, with an inversion-less polaritonic gain about 10 times larger than in equivalent GaN lasers. This combination of a very short injection section and a strong gain paves the way for compact microlasers with nonlinear functionalities for integrated photonics.
This dissertation is dedicated to the analysis of the physical mechanism determining the maximal optical power emitted by a III-N based laser diode. The aim of this study is to ascertain which factors affect the optical power of a laser diode and by that, try to increase the maximum optical power of the laser.
The first chapter is an introduction to the subject of the thesis, which begins with a description of the gallium nitride material and its properties (1.1). Then the historical outline of the development of gallium nitride (1.1.1) and optoelectronic devices based on the stimulated emission is presented: the laser diode (1.1.2), distributed feedback laser diode (1.1.3), superluminescent diode (1.1.4), and semiconductor optical amplifier (1.1.5). The next part of the first chapter is dedicated to the description of the radiative and non-radiative recombination processes in the semiconductor material (1.2). After it, the conditions which must be met to obtain optical gain are defined, considering the differences in the density of states in the volume material and quantum wells is and how such a distribution of states modifies the gain spectrum (1.3). After the introduction of the optical amplification, subsection 1.4 presents the basics of the most characteristic element of the laser diode – the optical resonator. In next two subsections, methods of confining the light in both vertical and lateral directions are shown. The next subsection (1.7) presents the physical mechanisms determining the maximum optical power emitted by a laser diode. The analyzed parameters include: power distribution mechanisms in laser diodes, threshold current/threshold current density, slope efficiency, carrier injection efficiency, mirror losses, internal optical losses, leakage of the optical mode into the substrate, thermal resistance and roll-over. The first chapter ends with a subsection describing the principles of the operation of a semiconductor optical amplifier.
The second chapter describes the measurement systems and measurement methods that I have used within this work. Due to the fact that this study consisted, among others, in creating a multifunctional workstation for measuring a number of basic and advanced parameters of laser diodes in various configurations and methods of assembly, the chapter begins with the specification of the requirements that must be met by the workstation. Then, the measurement methods are presented, their possibility and limitation are discussed. These methods include the measurements of: optical power-current-voltage characteristic (2.1), high-resolution electroluminescent spectra (2.2), gain spectra (2.3), stability of the spectrum and tuning laser diodes using the outer cavity (2.4), the thermal resistance (2.5), the near field (2.6) and optical signal amplification by an optical semiconductor amplifier (2.7). In addition, in the case of measurement of the gain spectrum, a detailed description of this method is presented. Also, an extensive analysis of the influence of the measurement system and measurement technique – resulting from hardware limitations (CCD camera noise and the finite resolution of the monochromator) – on the obtained results is carried out. Moreover, subsection 2.3.4 describes the effect of the resonator length on the depth of longitudinal mode oscillation, which is also crucial in measurements of the internal optical losses. The section of the measurement of the gain spectrum ends with a comment and comparison of the Hakki-Paoli method (used here) with other methods of determining the gain spectra.
Although the measurement results are already presented in the first and second chapter, they only serve as an aid in the description of a phenomenon/problem or as an example of a measurement result. The systematic study and analysis of the measurement results is presented starting from the third chapter. Chapter 3 consists of the results of research focused on the influence of the substrate, epitaxial structure and active region design on the optoelectronic parameters of the laser diode. The results are presented in such a way, which reflects the order of decision making, which are done during the growth and processing. The chapter begins with the study of the possibility of thinning the lower AlGaN cladding layer by the use of the substrate with increased concentration of the dopant (3.1). Then, the surprising results of research on the influence of the GaN substrate misorientation on the optoelectrical parameters of the laser diode are presented (3.2). Two cases of the off-cut angle are analyzed: the impact of the native misorientation of the substrate towards m¬-plane and the effect of the off-cut angle obtained by modification of the surface of the substrate towards a-plane. The results show that the off-cut angle has a great impact on the performance of the laser diode. Subsection 3.3 presents the results of the investigation of a unique laser diode structure based on the refractive index gradient. The investigation includes the results obtained by initial design (3.3.1) and in the improved design (3.3.2). In addition, this subsection includes the results of research on epitaxial structures that I proposed for semiconductor optical amplifiers/superluminescent diodes and laser diodes with reduced internal optical losses (3.3.3). Moreover, in subsection 3.3.4, there are results that present the effects of fabrication of the laser diode with an asymmetric waveguide. Subsection 3.4 shows the results of the investigation of the UV-A emitting laser diodes, in which the effect of doping of the lower waveguide on the parameters of the laser diode is tested. Then, in subsection 3.5, the influence of different types of quantum wells on the quantum confined Stark effect and differential gain is analyzed. The next subsection (3.6) contains the results of very important research of the influence of the position of the electron blocking layer in the laser diode structure and the level of doping on the internal optical losses and injection efficiency. The chapter ends with a study of the mechanism of fast degradation of laser diodes emitting in the UV-A range and the comparison of degradation of the MBE and MOVPE grown blue laser diodes, exposed to external laser light with a wavelength of 355 nm. In addition, this section includes a presentation of the unusual degradation of the laser diode mirrors associated with the chlorine contamination.
Chapter 4 is dedicated to the novel III-N-based emitters (distributed feedback laser diodes, tapered laser diodes and semiconductor optical amplifiers). This chapter presents not only the obtained optoelectrical parameters of these innovative and unique devices, but also includes an analysis of additional limitations of the maximum optical power resulting from their unusual design. In the case of distributed feedback laser diodes, their unique feature is the highly stabile completely single-mode emission (longitudinal, vertical and transversal) and an additional limitation is the overlap of the gain spectrum with spectral feedback of the periodic structure. In the case of the taper laser diode, its unique features are high brightness, reduced optical power density on the mirror and single vertical mode. On the other hand, the limitation is the increase in optical losses due to the instability of the cavity associated with the variable geometry of the waveguide. In the case of semiconductor optical amplifiers, a unique feature is the high amplification of the external optical signal and their limitation is the saturation of the gain.
The fifth chapter presents the results of research of the influence of thermal properties on the performance of laser diodes. This chapter begins with the analysis of the influence of the ambient temperature on the parameters of the laser diode (5.1), especially in terms of the T0 and T1 parameters. Moreover, the change of the differential gain and internal optical losses with temperature is also investigated. In the next subsection (5.2), the analysis is dedicated to the thermal properties of multi-emitter laser arrays and the effectiveness and usefulness of such device configuration in terms of the maximum optical power emitted from laser bars. Moreover, subsection 5.2.1 presents a non-trivial example of a problem with non-uniformity of laser array. The next subsection (5.3) includes the results of research on the influence of the geometry of the upper electrical contacts on the maximum optical power emitted by the laser diode. The shape of the upper contact, its thickness and width (but also the chip itself) are analyzed. Subsection 5.4 presents the results and analysis of laser diode parameters for various laser chip lengths and ridge widths. Chapter 5 ends with subsection 5.5 presenting the influence of various housings on the performance of a laser diode.
Chapter 6 summarizes the results of the research work along with the specification of the most interesting and most important results and conclusions.
Almost all research works (measurements/data processing and analysis of results), graphics, diagrams and photos presented in this paper were made by myself. All results that are not authored by me have been used with the consent of the author or authors and each result/analysis has appropriate information on who is the author of the result/analysis. Moreover, I showed the received support in the acknowledgments at the beginning of this thesis. All used quotes contain a relevant reference. In the case of Fig. 1.1b, the scheme is used under a CC BY 4.0 license.
Modal absorptions in laser-like heterostructures containing InAs self-assembled quantum dots (QDs) and InGaAs quantum well-dots (QWDs) have been studied. The evaluation of photoresponse as a function of waveguide length has allowed us to determine per-layer modal absorptions of 69 and 13 cm
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for the ground state optical transitions of QWDs and QDs, respectively. The values of the modal absorption can be used as a measure of the maximal (saturated) modal gain. To compare quantum heterostructures with different dimensionality we have introduced the layer gain constant, a parameter characterizing the light transmittance through the absorbing or gaining layer. We have shown that the QWD layer gain constant significantly exceeds quantum well and quantum dot ones.
This Letter reports on the temperature-dependent optical gain and absorption features, including the quantum confined Stark effect, of an InAs/InGaAs quantum-dash laser directly grown on a (001) Si substrate, with the lasing wavelength within the range of 1.5–1.6 μm. The maximum optical net gain was 22 cm⁻¹, and the internal optical loss was ∼–17 cm⁻¹ at 20 °C. Measurements as a function of injection level indicate that while the required current densities are still high, the intrinsic performance is significantly better than that of similarly operated InAs quantum dots operating at 1.3 μm, and further effort on growth could be made to reduce the internal optical losses and non-radiative current density. Optical modal absorption spectra were measured as a function of reverse bias from 0 V to 6 V, and a 40 nm redshift was observed in the absorption edge due to the quantum confined Stark effect, suggesting potential applications of these materials in electro-absorption modulators grown on silicon.
Among Professor Arthur Gossard’s many contributions to crystal growth are those resulting in important improvements in the quality and performance of quantum-well and quantum-dot semiconductor lasers. In celebration of his 85th birthday, we review the development of a semiconductor laser theory that is motivated and guided, in part, by those advances. This theory combines condensed matter theory and laser physics to provide understanding at a microscopic level, i.e., in terms of electrons and holes, and their interaction with the radiation field while influenced by the lattice.
This letter reports on InP/GaInP quantum dot mode-locked lasers emitting in the 730 nm wavelength region, extending the spectral range of previously reported monolithic mode-locked edge-emitting lasers. Modal gain and absorption measurements were used to identify a relatively broad spectrum which is utilised to support passive mode-locking in a monolithically integrated two-section ridge laser. The conditions for mode-locking were explored by varying the current to the gain section and reverse bias to the absorber section. For a total cavity length of 3 mm, the shortest pulse train observed was 6 ps in duration with a repetition rate of 12.55 GHz.
Heteroepitaxial growth of semipolar laser diodes (LDs) on foreign substrate is extremely challenging but crucial to reduce the cost of semipolar bulk GaN substrates. In this work, we demonstrate the first efficient semipolar 410 nm violet LDs grown on high-quality low-cost semipolar GaN/sapphire substrates. The fabricated semipolar LD exhibits a high output power of more than 900 mW at 1.6 A, and a wall-plug efficiency of 5.6% under pulsed operation. The analysis also quantitatively confirms that the high density of defects and high non-radiative recombination rate in the active region account for the high transparency current observed in the devices.
The work presented in this dissertation focus on the development of InP-based semiconductor vertical-cavity lasers, based on quantum nanostructures and emitting at the telecom wavelengths (1550-1600 nm). A new technological process for the realization of compact VCSELs is described. This process (named TSHEC) has been employed to realize optically-pumped VCSELs, integrated onto a host Silicon platform, with good performances. The same process has been adapted to develop an electrically-driven version of VCSELs: a preliminary study of the confinement section based on a InGaAs-BTJ is presented, together with the development of a mask set. Thanks to the development of the liquid crystals μ-cell technology (in collaboration with LAAS, IMT Atlantique et C2N), we realized a tunable photodiode at 1.55 μm, and a tunable VCSEL is currently under development. This work also presents the first realization of a 1.6 μm- emitting optically-pumped quantum dashes-based VECSELs, and its characterization in multi-mode and single-frequency regime. Finally, the realization of an experimental setup for the investigation of the coupling between two orthogonal eigenstates of a bi- frequency 1.54 μm-emitting SQW-VECSEL has been conceived and realized. This setup, which allowed the direct quantification of the coupling constant on such a device, in the near future will allow performing the same study on anisotropic structures like quantum dashes or quantum dots, with the objective of studying the inhomogeneous broadening effect observed in these gain regions.
We investigated the optical gain properties and lasing characteristics of a pulsed electrically pumped laser structure, which consists of a single layer of self-assembled InP quantum dots (QDs) assembled in (Al
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Ga
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)
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In
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P barriers. The optical gain and absorption spectra were obtained by analyzing the amplified spontaneous emission as a function of the excitation length. At a current density of 1.2 kA/cm
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, an internal optical loss value of 5 ± 2 cm
$^{{-1}}$
and a peak modal gain of 39.3 cm
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corresponding to a material gain of approximately 2675 cm
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per QD layer were determined at room temperature. For a 2.24-mm-long laser with uncoated facets emitting at 660 nm, a low threshold current density of 281 A/cm
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and an external differential quantum efficiency of 34.2% were determined. The internal quantum efficiency of 66% and the transparent current density of 65.8 A/cm
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for a single layer of QDs were also demonstrated.
Quantum dot (QD) laser diodes, where the active region consists of wetting-layer-free InAs QDs, were demonstrated by block copolymer (BCP) lithography and subsequent selective area metal-organic vapor phase epitaxy, which results in the QD density of ∼4 -10 ¹⁰ cm ² . In this work, we show that an In 0.1 Ga 0.9 As QW located in close proximity to the wetting-layer-free InAs QDs leads to an enhanced carrier injection into the QDs, allowing for lasing at room temperature (RT). Devices employing InAs QDs grown with and without In 0.1 Ga 0.9 As QW carrier collection layer exhibit lasing at 80 K, while only the lasers grown with the In 0.1 Ga 0.9 As QW exhibits lasing at RT. Driving current-dependent electroluminescence measurements reveals a low carrier injection into the QDs in the device grown without In 0.1 Ga 0.9 As QW carrier collection layer. In addition, the appearance of lasing emission, corresponding to high energy transitions, indicates a relatively flat gain profile for this InAs QD active region grown by SA-MOVPE. In addition, EL measurements at 80 K on devices employing varying thickness (d InAs ) QDs indicate the peak emission wavelength varies from 920 nm (d InAs ∼ 1.5 nm) to 974 nm (d InAs = 3 nm), although the devices employing d InAs ∼ 3 nm did not exhibit lasing, possibly because of a significant degree of strain relaxation for this QD thickness.
Lowering the threshold gain of InAs quantum dot lasers grown on Silicon, significantly extends device lifetime. Measurements on degraded devices show increased optical mode loss is responsible for degradation and a consequent shortening of lasing wavelength.
InGaN-based laser diodes (LDs) grown on the semipolar (202¯1¯) plane of GaN offer advantages arising from predicted higher gain compared to c-plane devices. However, the performance of semipolar devices has been limited by low injection efficiency. In prior work, this has been proposed to be caused by an inefficient AlGaN electron blocking layer (EBL). A high oxygen level in the EBL found in previous work could cause compensation of the p-type dopants. In this work the oxygen impurities were eliminated via optimizations during the metalorganic chemical vapor deposition (MOCVD) growth. Optimization of the V/III ratio and an increase in the growth temperature during the AlGaN growth was found to reduce the oxygen incorporation. Additionally, residual sources of oxygen were removed from the MOCVD growth process. These combined steps reduced the peak oxygen level in the AlGaN from 5 × 10¹⁸/cm³ to 6 × 10¹⁷/cm³ as measured by secondary ion mass spectroscopy. LDs were fabricated with and without these modifications to clarify the effect of the reduced oxygen. The threshold current and differential efficiency showed improvement with reduced oxygen. The internal loss and gain of these LDs were measured using the segmented contact method, confirming that the injection efficiency was boosted from 60% with high oxygen to 80% with low oxygen.
Gain spectra for GaAs double-heterostructure junction lasers have been obtained with high resolution. This is accomplished by using an automated data aquisition system to analyze the Fabry-Perot resonance modulation in the spontaneous emission spectra. For active regions doped with Ge at a level of 4×10<sup>17</sup> cm<sup>-3</sup>, the gain in the TE polarization at a fixed wavelength increases linearly with current, below lasing threshold. However, the peak gain (at a variable wavelength) increases slightly faster than linearly with current. The photon energy at which gain is a maximum increases logarithmically with current. Gain in the TM polarization depicts the same general behavior as that for the TE case, except that it is slightly less than the TE gain. It is concluded that for this particular doping the spectral gain characteristics are intermediate between those for undoped and heavily doped active regions. Above the threshold for lasing in the TE mode the TE gain spectra are well saturated, with new fine details revealed in the saturated spectra. On the other hand, gain in the nonlasing TM polarization is not well saturated above threshold, with marked differences in gain between high and low photon energies relative to the TE lasing energy.
The widely used technique for determining optical gain and absorption from single-pass, amplified spontaneous emission (ASE) is analyzed with particular attention given to active-volume saturation and three-dimensional effects. The results for homogeneously broadened media show that the population inversion and hence the gain in the active region is diminished by high levels of spontaneous and ASE. The externally measured gain is further reduced for large output collection apertures that admit incompletely amplified off-axis rays. Criteria are given for the excitation and detection geometries required to ensure accurate gain measurement. The simultaneous measurement of the equilibrium absorption coefficient can be made under less restrictive conditions.
Premature catastrophic degradation in sawn-cavity (GaAl)As/GaAs heterostructure lasers can be attributed to the presence of internally circulating modes. When lasers were selected for good near-field uniformity and assumed freedom from internally circulating modes, good agreement was obtained between the power at catastrophic degradation and the effective optical width perpendicular to the junction.
Measurement of the spontaneous emission and gain spectra provides a complete characterization of a semiconductor gain medium, however, this requires the observation of emission in two directions to avoid amplification of the spontaneous emission spectrum. We show that both the gain spectrum and the true spontaneous emission spectrum can be obtained from amplified spontaneous emission (ASE) spectra measured from the end of a segmented-contact device. The spontaneous emission spectra agree with spectra measured through a top contact window. If the carrier populations are fully inverted at low photon energy, it is possible to convert the ASE-derived spontaneous emission into real units. © 2002 American Institute of Physics.
Using experimental measurements of the gain–current characteristic as a function of temperature in InGaAs quantum-dot lasers, we demonstrate that it is the population of wetting-layer states that leads to a saturation of the population inversion in dot states and hence to the saturation of gain in a quantum-dot laser. At 300 K, the maximum modal gain for a three-layer structure is reduced from 53 to 14 cm−1. © 2002 American Institute of Physics.
Use is made of a numerical simulation of the light—current characteristic to examine the errors which may arise in the determination of the optical mode loss (αi) from the cavity length dependence of the external differential efficiency (ηextd). In particular, we focus on the effects of incomplete Fermi level pinning and carrier leakage, and show that αi can only be determined correctly if ηextd is determined under conditions where it is invariant both with current level and temperature. © 1997 American Institute of Physics.
We describe a technique for the measurement of optical gain and loss in semiconductor lasers using a single, multisection device. The method provides a complete description of the gain spectrum in absolute units and over a wide current range. Comparison of the transverse electric and transverse magnetic polarized spectra also provides the quasi-Fermi-level energy separation. Measurements on AlGaInP quantum well laser structures with emission wavelengths close to 670 nm show an internal loss of 10 cm−1 and peak gain values up to 4000 cm−1 for current densities up to 4 kA cm−2. © 1999 American Institute of Physics.
Ingredients. A Phenomenological Approach to Diode Lasers. Mirrors and Resonators for Diode Lasers. Gain and Current Relations. Dynamic Effects. Perturbation and Coupled--Mode Theory. Dielectric Waveguides. Photonic Integrated Circuits. Appendices (16). Index.
Experimental determinations have been made of the peak optical gain as a function of spontaneous recombination current density for GaAs quantum wells of width 25 and 58 Å bounded by AlGaAs barriers. These data were obtained from measurements of spontaneous emission spectra, observed through narrow windows in the 50‐μm‐wide contact stripes of oxide isolated lasers, using only a single reference value of the optical absorption coefficient above the band edge to calibrate the measurements in absolute units. These results are in good agreement with gain‐current curves calculated using a model which includes unintentional monolayer well width fluctuations, band‐gap narrowing and intraband carrier‐carrier scattering. The characteristic intraband scattering time is calculated from first principles as a function of electron energy and carrier density on the basis of a 2‐dimensional Auger‐type process. This lifetime gives a much better representation of our observed spontaneous spectra than a lifetime which is simply dependent upon carrier density. The comparison between experiment and model calculation involves no adjustable parameters. For the 58‐Å‐wide wells there is a difference between the experimental and calculated gain‐current curves at low values of gain. We show that this is a consequence of applying the Einstein relations to a broadened spectrum in the process of deriving the gain from the observed spontaneous emission spectrum. A direct comparison of the shapes of experimental and calculated spontaneous emission spectra at several injection levels provides a more rigorous, yet equally valid, verification of the computer model.
A new method for measuring absorption and gain spectra of lasers is presented. These spectra are deduced from measurements of spontaneous emission spectra at different laser currents supplemented by measurements of the laser line energy and the differential quantum efficiency. The spontaneous emission emerged from the side of the laser after traveling through a transparent cladding layer. At each current, the bias energy eV is determined. A simple theoretical model is used to convert eV to minority carrier density. The method is based on the application of general relations between the rates of spontaneous emission, stimulated emission, and optical absorption. A new general proof of these relations is presented. The gain versus carrier density relation at the laser line energy is measured for four samples having different active layer doping or Al composition. Gain increased superlinearly with carrier density in undoped and n‐type samples and increased slightly sublinearly in a p‐type sample. The losses at low carrier densities ranged from 100–200 cm<sup>-1</sup>. For one undoped sample, the changes in the absorption edge caused by the electron and hole densities increasing from 5×10<sup>16</sup> to 1.1×10<sup>18</sup> cm<sup>-3</sup> were deduced by comparing the measured changes with a model calculation. It was found that the exponential broadening increased 20%, that the energy gap decreased 12–16 meV, and that the strength of optical absorption at low energies decreased by about a factor of 1.4.
A simple technique for determining the gain spectra of semiconductor lasers from measurements of the emission spectra of the laser is presented. The technique is insensitive to the response function of the device used to determine the emission spectra of the laser. Accurate estimates of the gain can be obtained from data which have been convolved with an instrument response function of ≲0.5 Å FWHM for a cavity free spectral range of 2.5 Å. Two applications of the gain data obtained by the technique are presented.
The maximum efficiencies of ideal solar cells are calculated for both single and multiple energy gap cells using a standard air mass 1.5 terrestrial solar spectrum. The calculations of efficiency are made by a simple graphical method, which clearly exhibits the contributions of the various intrinsic losses. The maximum efficiency, at a concentration of 1 sun, is 31%. At a concentration of 1000 suns with the cell at 300 K, the maximum efficiencies are 37, 50, 56, and 72% for cells with 1, 2, 3, and 36 energy gaps, respectively. The value of 72% is less than the limit of 93% imposed by thermodynamics for the conversion of direct solar radiation into work. Ideal multiple energy gap solar cells fall below the thermodynamic limit because of emission of light from the forward‐biased p‐n junctions. The light is radiated at all angles and causes an entropy increase as well as an energy loss.
We report a new technique for measuring the stimulated emission spectrum and optical gain of semiconductor materials. Amplified spontaneous emission is used to determine the gain factor by relating the measured variation in light output to variation in the length of the excitation beam. Results for CdS crystals at 2°K are presented that indicate net gains as high as 160 cm<sup>-1</sup> at λ = 4907 Å are possible with ∼ 12‐MW/cm<sup>2</sup> optical pump power density from a nitrogen laser.
We have analyzed the internal differential efficiency of
quantum-well lasers in terms of current spreading, carrier injection
into the well, and the radiative efficiency within the well. We quantify
the first two of these processes by extracting information from
spontaneous emission measurements as a function of device length,
current, and temperature. We show that the carrier injection efficiency
is responsible for the temperature dependence of the external
differential efficiency of GaInP quantum-well (QW) lasers by comparing
values from the slope of the laser power output versus current
characteristic with our experimental values for current spreading and
injection efficiency
A new method for determining gain spectra is presented. The
amplified spontaneous emission is measured and spectrally resolved in
both TE and TM polarisations; it is dependent on the current injected
into the contact stripes of variable length on the laser structures. The
maximum gain and internal losses correspond well with the results of
broad area lasers
The widely used technique for determining optical gain and absorption from single-pass, amplified spontaneous emission (ASE) is analyzed with particular attention given to active-volume saturation and three-dimensional effects. The results for homogeneously broadened media show that the population inversion and hence the gain in the active region is diminished by high levels of spontaneous and ASE. The externally measured gain is further reduced for large output collection apertures that admit incompletely amplified off-axis rays. Criteria are given for the excitation and detection geometries required to ensure accurate (∼10-percent) gain measurement. The simultaneous measurement of the equilibrium absorption coefficient can be made under less restrictive conditions.
The purpose of this paper is to explore the dimensionality of the
optoelectronic properties of quantum-well and dot systems by expressing
carrier distributions in the confinement directions in terms of envelope
functions rather than assuming that carriers are localized to the
geometrical extent of the confining potential. The conclusions apply to
an ideal two-dimensional (2-D) system or a structure where only the n=1
electron and hole subbands are populated. We show that optical
absorption normal to the plane of a QW cannot be expressed as an
absorption coefficient but should be specified as a fraction of light
transmitted or absorbed per well. The modal gain for light propagating
along the plane of a QW does not scale with well width and the variation
of the material gain inversely proportional to the well width is a
consequence of the definition of the confinement factor and has no
independent physical significance. Coupling to the optical mode can be
specified as a mode width without the need to assume the gain medium is
localized in the well. Optical absorption and gain by quantum dots
should be expressed as a cross section per dot. The radiative
recombination current should be expressed in terms of a two-dimensional
recombination coefficient and use of an equivalent three-dimensional
coefficient introduces an artificial dependence on well width which can
lead to errors in the comparison of QW systems. We provide an analysis
of experimental data for optical absorption in GaAs wells and show that,
using the correct dimensional forms, it is straightforward to use this
to estimate modal gain and the recombination coefficient
IEEE J. Quantum Electron.
- P S Cross
- W G Oldham
IEEE J. Select. Topics Quantum Electron.
- P M Smowton
- P Blood
- Ieee J Select